Platinum group metals are often employed as catalysts in automobile catalytic converters. Because platinum group metals are relatively expensive and are often obtained from sources outside the United States, it is advantageous to recover platinum group metals from such catalyst materials. Because most catalyst materials containing platinum group metals are of a low grade, direct dissolution and refining of the platinum group metals is not feasible and the catalyst materials must first be processed to obtain a platinum group metal-containing concentrate.
Both hydrometallurgical and pyrometallurgical methods are currently employed to recover platinum group metals from catalyst materials and the like. For example, strong sulfuric acid leaching has been used to dissolve the gamma alumina substrate of pellet-type catalysts and yield a platinum group metal concentrate. Monolith catalysts have been leached with sulfuric acid to dissolve the gamma alumina wash coat from the cordierite substrate. After washing, the platinum group metals are dissolved with aqua regia, and the resulting solution is treated to recover a platinum group metal concentrate. Dry chlorination has also been shown to be effective for transporting metal chlorides from the support. Pyrometallurgical methods for processing have included plasma fusion with an iron collector metal, melting the catalyst with copper as a collector metal, and charging the materials directly to a smelter to recover the platinum group metals in the normal operating processes.
Excluding the method of platinum group metal recovery by direct charging to smelters, the estimated recoveries for platinum group metals in these prior art processes range from 80 to 94 percent for platinum, 80 to 96 percent for palladium and 65 to 90 percent for rhodium. However, reagent consumption and severe corrosion cause the hydrometallurgical processes to be disadvantageous. On the other hand, high energy consumption and contamination of the platinum group metals with impurities such as nickel cause the pyrometallurgical processes to be disadvantageous. Thus, new methods for recovering platinum group metals from catalyst materials and the like which overcome the disadvantages of prior art methods are needed.
Cyanide has been used to dissolve platinum group metals from ores at temperatures up to 120.degree. C. with only limited success owing to low extractions and high cyanide consumption, Dawson, "Testwork on Samples of Oxidized Ore from the Potgietersrus Prospect," Mintek, South Africa, No. M59D, 1982, 26pp. Dawson disclosed that only 30 percent of the platinum and 75 percent of the palladium were dissolved. Additionally, thermal treatment of solutions containing free and complexed cyanides of Fe, Ni, Ag, Cd, Zn and Cu has been disclosed as effective for cyanide disposal, Tan et al, Plt. Srf. Fn., Vol. 74, No. 4, April 1987, pp. 70-73.